1. Field of the Invention
The invention relates to a process for combusting ammonia-containing gases which also contain hydrogen sulfide, in the thermal zone of a sulfur recovery unit.
2. Description of the Prior Art
The recovery of elemental sulfur from hydrogen sulfide-containing gases by means of the Claus reaction is known and different operation modes for carrying out this reaction are being applied in practice.
In one of these cases the Claus reaction is carried out by partially combusting the hydrogen sulfide-containing gases with air in a thermal zone to form a gas mixture containing sulfur dioxide (SO.sub.2) and hydrogen sulfide (H.sub.2 S) in a preferred ratio of 1:2, whereupon the gases are cooled after leaving the thermal zone in order to condense the sulfur vapor formed and recover the sulfur as liquid sulfur. Subsequently, the gases are conducted to one or more catalytic reaction zones where an extra quantity of sulfur is formed. Before introduction into a catalytic zone the gases are first brought to the desired reaction temperature and after leaving this zone they are re-cooled by recovery of the sulfur formed.
The hydrogen sulfide-containing gases mostly originate from desulfurization processes in a refinery or from natural gas from which the acid gas components are removed in gas purification units by means of absorption in certain absorbents. After regeneration of the usually aqueous absorbent used, a gas is obtained which contains the hydrogen sulfide in enriched concentration.
A great many if not most crude oils processed in a refinery contain nitrogen compounds in addition to sulfur compounds. In the large number of catalytic processes which are used for processing crude oils and employ hydrogen, ammonia (NH.sub.3) is obtained in addition to hydrogen sulfide. Hydrogen sulfide and ammonia are also formed in many processes which are not primarily intended to remove sulfur and nitrogen, such as thermal and catalytic cracking. Consequently, liquid or gaseous product streams containing said compounds are formed, which compounds must be removed from said streams. The removal of ammonia (and partly of hydrogen sulfide) from such streams, generally originating from hydrocarbon conversion processes, can be effected by washing with water at, for example, elevated pressures and reduced temperatures. Washing is mostly carried out with an abundant quantity of water, so that dilute solutions are formed which contain ammonia and, in addition, hydrogen sulfide. In view of the increasing legislation on environmental pollution abatement such water streams can no longer be discharged as such into open surface water streams.
As a result of the various other treatments in a refinery, in addition to the above-mentioned water stream, other waste water streams are formed which, combined with the above water stream, can be jointly referred to as "sour water". In order to render said polluted water stream suitable for discharge into open surface water steam-stripping is currently applied. This results in an acid stripper gas containing ammonia and hydrogen sulfide in addition to water vapor. The acid gas can then be conducted to a combustion furnace and after combustion be discharged to the environment through a stack. However, if the acid gas contains much hydrogen sulfide, the water pollution problem has been changed into an air pollution problem. A sulfur recovery unit being present, it might be possible to consider supplying this acid gas to said units in order to combust both the ammonia present and the hydrogen sulfide. Although various proposals have already been made to this end, the supply of ammonia-containing gas streams to a sulfur recovery unit presents difficulties in the operation of such units for a complex of reasons. If the mixing of the ammonia-containing gas with the hydrogen sulfide-containing gas and air is not accomplished to perfection, firstly the conversion of ammonia cannot be complete and secondly, as a result of local excess of oxygen, nitrogen oxides (NO.sub.x) can be formed from ammonia, which nitrogen oxides, it is assumed, catalytically promote the formation of sulfur trioxide (SO.sub.3) from sulfur dioxide. Said sulfur trioxide formed may subsequently cause serious dewpoint corrosion in colder parts of the unit or cause the formation of solid deposits of ammonium salts in those parts of the unit where the temperature has fallen sufficiently, such as in the sulfur condensers, pipelines and the like. This results in clogging of pipes, etc.
The present invention aims at providing a process enabling the ammonia-containing gases which also contain hydrogen sulfide to be combusted in a sulfur recovery unit without the occurrence of said drawbacks. Other objectives will appear from the following.